Knockout mouse technology has provided us with an excellent tool to study the effects of individual genes within an intact organism by adding and removing copies of wild type genes or replacing them with mutant forms. The challenge in Neurobiology will be to not only determine the overall effects on the gross morphology of the nervous system or its effects on animal behavior, but to elucidate the underlying changes at the single cell level. The additional challenges for the Visual Neuroscientist are, however that the cells in mouse retina are extremely small and for this reason are poorly characterized at the physiological level, and thus the majority of techniques used to study retinal circuitry and single cell physiology have been established for cold-blooded animals with large, easily identifiable cells. However, just these function analysis tools are required to bring gene technology to the next level. We propose for this study to combine the two main features, genetics and functional analysis, in one model system. In order to gain access to single, genetically modifiable retinal cells, which can be studied using single cell physiology and imaging, we are developing a transgenic frog system according to Amaya and Kroll (1999). This technique will be applied to further investigate the genotype of the neurotrophin receptor trkB-/- mouse. We have used the trkB-/- mouse to study neurotrophin dependent photoreceptor development and signaling (Rohrer et al., 1999), and have demonstrated that the elimination of trkB interferes with the normal developmental maturation of rods and their signaling to rod bipolar cells, with a presumed defect presynaptically, in the rod ribbon synapse. The obvious next question is whether there is a defect in synaptic transmitter release from the photoreceptors, and to answer this question requires single cell imaging and electrophysiology . The results from these studies will provide important information for evaluating the neuroprotective potential of neurotrophins in retinal disease. However, more importantly, it will provide avenues for studying gene defects at the single cell level not only in the retina, but in other systems as well.